Conventional methods of controlling operating modes in a circuit include the use of dedicated pins carrying different levels of voltage. For example, a typical printed circuit board may contain 12V, 5V, and 3V DC reference points to permit operation of mixed device logics. Typically, a charge pump, regulated power supply or other similar source of power is used to provide the required array of voltages. Such methods have been incorporated by manufacturers into a wide array of applications.
Multiple voltage reference points are also standard in many microelectronic systems where feature selection is accomplished by placing a HIGH or LOW level signal on a reference pin to indicate a desired operand, function or other selection. For example, the read/write select line on an integrated circuit chip may be toggled by using either a 5V or 0V DC level, one of which is associated with either a read or write function. An operational amplifier switch may be employed to sink and lift the line to either the HIGH or LOW signal, as needed.
An important design aspect is the control time associated with selecting or enabling a mode of operation. For many applications, a voltage level is measured as the differential between any two points on the system plane. Where the differential occurs over a ramping signal source, such as a shunt or stabilizing capacitor, the signal amplitude versus time may have a ramping characteristic. The control time of such a system, thus, becomes a function of the time it takes to ramp to a given triggering voltage within allowed tolerances.
In addition, in many applications a premium is placed on the amount of real estate consumed by the electronics. Today's systems are fitted on smaller and smaller footprints with the underlying devices consuming less space while providing more advanced functions as a result of higher package densities. The use of multiple voltage reference points adds to the space requirements of the overall design.
As such, an object of the present invention is to provide a current biased mode control circuit for selecting operating modes, features and functions in an electronic subsystem that has a relatively fast control time as compared with prior art methods and designs.
It is another object of the present invention to provide such a circuit with a variable current source having a range of values wherein each range is associated with a predefined mode of operation.
It is another object of the present invention to provide such a circuit with at least two mode switching devices for selecting associated modes of operation.
Other objects, features and advantages of the invention shall be apparent to those of ordinary skill in the art upon reference to the following detailed description taken in conjunction with the accompanying drawings.